When an ethylene homopolymer and copolymers of ethylene and an α-olefin having 3 to 20 carbon atoms prepared by polymerization using a transition metal catalyst such as metallocene catalyst or Ziegler-Natta catalyst (hereinafter referred to as ethylene polymers) in the same density region as conventional high-pressure low density polyethylene are molded into a film or sheet, it has more excellent mechanical strength such as tensile strength, tear strength or impact strength, and further excellent heat resistance, stress and scratch properties, optical properties and heat sealing properties as compared with high-pressure low density polyethylene.
The ethylene copolymers, however, have such a problem in that the fluidity represented by a ratio (MFR10/MFR2) of a melt flow rate (MFR10) under a load of 10 Kg at 190° C. to a melt flow rate (MFR2) under a load of 2.16 Kg at 190° C. is lower than that of conventional high-pressure low density polyethylene, and the moldability is not sufficient.
On this account, if ethylene polymers which have a high MFR10/MFR2 value and excellent fluidity at the same density region as conventional high pressure low density polyethylene can be prepared by polymerization with the transition metal catalyst, its industrial value will be very high.
At present, it is impossible for the high-pressure method to prepare low-density polyethylene having a Mw/Mn of lower than 4.0. The polymers having a lower Mw/Mn have high homogeneity and show that, after polymer molding, blocking or appearance failure is hardly induced. Accordingly, the production of low-density polyethylene having a Mw/Mn ratio of lower than 4.0 and a high MFR10/MFR2 ratio like the high-pressure low-density polyethylene at the same density region as high-pressure low-density polyethylene (0.921 to 0.929 g/cm3) has been desired.
Further, ethylene polymers in the lower density region than that of conventional high-pressure low-density polyethylene are useful as a modifier or compatibilizing agent, but it is impossible for the ethylene polymer in this density region to improve the fluidity by increasing the MFR10/MFR2 ratio thereof.
The method of increasing the MFR10/MFR2 ratio by increasing the Mw/Mn value is known generally. The method, however, has such a problem that increasing the Mw/Mn value to more than 11 lowers the homogeneity of the polymer, thereby resulting in occurrence of blocking or appearance failure. On this account, it is desired that the Mw/Mn value be less than 10. However, it is very difficult for the ethylene polymers in the region of the Mw/Mn value of less than 10 to increase the MFR10/MFR2 ratio until the same region of high-pressure low-density polyethylene (the MFR10/MFR2 ratio being from 16.2 to 50).
Furthermore, ethylene polymers in the higher density region than that of conventional high-pressure low-density polyethylene are molded into vessels or films and then used suitably. However, ethylene polymers having the same fluidity as that of conventional high-pressure low-density polyethylene cannot be prepared.
In the light of the foregoing background, the preparation of ethylene polymers having a high MFR10/MFR2 ratio and excellent fluidity by polymerization with the transition metal catalyst has been studied.
JP-A-2-276807/1990 discloses that copolymerization of ethylene and an α-olefin of 3 to 20 carbon atoms in the presence of an olefin polymerization catalyst comprising a specific hafnium compound and organic aluminum oxy compound produces ethylene copolymers having the MFR10/MFR2 ratio increased until that of conventional high-pressure low-density polyethylene in the lower density region than that of conventional high-pressure low-density polyethylene, even if the Mw/Mn value is less than 10.
The present inventors found that the use of an organic boron compound with a specific organic hafnium compound in place of the organic aluminum oxy compound is more effective in preparation of ethylene polymers in the lower density region than that of conventionally high-pressure low-density polyethylene. Thus, the preparation process of the present invention has been accomplished.
Further, the preparation of, at the higher density region than that of conventional high-pressure low-density polyethylene, ethylene polymers having the MFR10/MFR2 ratio increased to that of conventional high-pressure low-density polyethylene has been desired. For increasing the MFR10/MFR2 ratio with keeping Mw/Mn of less than 10, it is generally known that it is effective to introduce a long chain branch into a polymer main chain. In this case, introduction of the long chain branch into the polymer main chain lowers the density of the ethylene polymer, so that this procedure is effective only in the low density region as described in JP-A-2-276807/1990 and it is difficult to apply this procedure in the high density region. JP-A-7-500622/1995 discloses that a MFR10/MFR2 ratio is increased even in a higher density region than that of conventional high-pressure low-density polyethylene with keeping Mw/Mn of less than 10 by the combined use of an organic boron compound and a specific organic titanium compound. Though the MFR10/MFR2 ratio is increased, it does not reach to the same level (MFR10/MFR2 ratio in the region of 16.2 to 50) as that of conventional high-pressure low-density polyethylene. Further, JP-A-7-500622/1995 discloses that the preparation of an ethylene polymer in the high density region is more difficult than that of an ethylene polymer in the low density region. In the examples thereof, an ethylene polymer having a high density as described above is prepared by carrying out polymerization at a higher temperature. That is, in the examples of JP-A-7-500622/1995, polymerization is carried out at 200° C. which is much higher than usual polymerization temperatures in order to increase the MFR10/MFR2 ratio to 16.1.
The present inventors found an ethylene polymer which MFR10/MFR2 ratio has been increased to the same extent (MFR10/MFR2 ratio in the range of 16.2 to 50) of conventional high-pressure low-density polyethylene with keeping Mw/Mn of lower than 10 in the higher density region than that of conventional high-pressure low-density polyethylene. Thus, the first ethylene polymer of the present invention has been accomplished.
The preparation of an ethylene polymer which MFR10/MFR2 ratio has been increased to the same as that of conventional high-pressure low-density polyethylene with keeping the Mw/Mn of less than 4.0 in the same density region as conventional high-pressure low-density polyethylene (0.921 to 0.929 g/cm3) has been desired. JP-A-7-500622 discloses that a MFR10/MFR2 ratio is increased in the same density region than that of conventional high-pressure low-density polyethylene with keeping the Mw/Mn of less than 4.0 by the combined use of an organic boron compound and a specific organic titanium compound. However, a polyethylene having a MFR10/MFR2 value of higher than 10.6 still is not prepared.
The present inventors found an ethylene polymer which MFR10/MFR2 ratio has been increased to the value including the value of the MFR10/MFR2 ratio of conventional high-pressure low-density polyethylene with keeping the Mw/Mn of lower than 4.0 even in the higher density region than that of conventional high-pressure low-density polyethylene. Thus, the second ethylene polymer of the present invention has been accomplished.
Further, conventional ethylene polymers have been desired to be more improved on their high-speed moldability. The present inventors found an ethylene polymer having more excellent high-speed moldability than that of conventional ethylene polymers. Thus, the present invention has been accomplished.
Moreover, the present inventors found the combined use of the organic boron compound and a specific hafnium compound in place of an organic aluminum oxy compound is more effective in preparation of an ethylene polymer having excellent high-speed moldability. Thus, the present invention has been accomplished.
The present inventors, further, have accomplished the invention of molded articles prepared by using the ethylene polymers.